The invention relates to a method for recuperating energy from an exhaust gas flow of an internal combustion engine and a motor vehicle, in which such an exhaust gas flow is available and can thus be used to recuperate heat therefrom.
The exhaust gas coming from an internal combustion engine has a high temperature and contains heat energy which could be recuperated for example in the form of electrical energy via a so-called Clausius-Rankine cycle. Herein, a working fluid usually water, is alternately evaporated at a high pressure and condensed at a low pressure. The high pressure is reduced in an expansion machine, in particular a turbine, while generation work, so that electrical energy can be obtained. After condensation in a condenser, the working fluid, which is now liquid, is pressurized by means of a pump and supplied to an evaporator. There, the liquid working fluid is heated by another fluid via indirect heat transfer, in which the other fluid is separated from the working fluid via a heat conductive wall. It is conceivable that the other fluid is the exhaust gas of the internal combustion engine. Up to now there are however difficulties for the realization in that the exhaust gas mass flow is variable and depends in particular on the momentary performance of the internal combustion engine. It is thus not sensible to circulate the working fluid in the Clausius-Rankine cycle at a constant volumetric flow. A mass flow control of the working fluid was already considered, which should take place in dependence on measuring values obtained at the working fluid, e.g. in dependence on temperature and mass flow or pressure of the working medium. Due to the inertness of such a control, which is caused by the relation of the internal volume of the evaporator to the mass flow, a stable control of the type mentioned has proved to be impossible.
In EP 1 333 157 A1, it is indeed described that the exhaust gas flow from an internal combustion engine is used as energy source for a Clausius-Rankine cycle. Two circuit systems are provided hereby, where the working fluids are distinguished from each other by their boiling point. The pump capacity can be adjusted in a variable manner in at least one of the circuits. In EP 1 333 157 A1 it is stated that the efficiency of the recovery of heat energy from the exhaust gas flow can be adjusted in a maximum manner.
It is the object of the present invention to provide a method for obtaining energy from an internal combustion engine, which can be implemented in a practical but relatively simple manner.